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BBC Micro Elite

Dashboard: SP2

Name: SP2 [View in context] Type: Subroutine Category: Dashboard Summary: Draw a dot on the compass, given the planet/station vector
Draw a dot on the compass to represent the planet or station, whose normalised vector is in XX15. XX15 to XX15+2 The normalised vector to the planet or space station, stored as x in XX15, y in XX15+1 and z in XX15+2
.SP2 { LDA XX15 \ Set A to the x-coordinate of the planet or station to \ show on the compass, which will be in the range -96 to \ +96 as the vector has been normalised JSR SPS2 \ Set (Y X) = A / 10, so X will be from -9 to +9, which \ is the x-offset from the centre of the compass of the \ dot we want to draw. Returns with the C flag clear TXA \ Set COMX = 195 + X, as 186 is the pixel x-coordinate ADC #195 \ of the leftmost dot possible on the compass, and X can STA COMX \ be -9, which would be 195 - 9 = 186. This also means \ that the highest value for COMX is 195 + 9 = 204, \ which is the pixel x-coordinate of the rightmost dot \ in the compass... but the compass dot is actually two \ pixels wide, so the compass dot can overlap the right \ edge of the compass, but not the left edge LDA XX15+1 \ Set A to the y-coordinate of the planet or station to \ show on the compass, which will be in the range -96 to \ +96 as the vector has been normalised JSR SPS2 \ Set (Y X) = A / 10, so X will be from -9 to +9, which \ is the y-offset from the centre of the compass of the \ dot we want to draw. Returns with the C flag clear STX T \ Set COMY = 204 - X, as 203 is the pixel y-coordinate LDA #204 \ of the centre of the compass, the C flag is clear, SBC T \ and the y-axis needs to be flipped around (because STA COMY \ when the planet or station is above us, and the \ vector is therefore positive, we want to show the dot \ higher up on the compass, which has a smaller pixel \ y-coordinate). So this calculation does this: \ \ COMY = 204 - X - (1 - 0) = 203 - X LDA #&F0 \ Set A to a 4-pixel mode 5 byte row in colour 2 \ (yellow/white), the colour for when the planet or \ station in the compass is in front of us LDX XX15+2 \ If the z-coordinate of the XX15 vector is positive, BPL P%+4 \ skip the following instruction LDA #&FF \ The z-coordinate of XX15 is negative, so the planet or \ station is behind us and the compass dot should be in \ green/cyan, so set A to a 4-pixel mode 5 byte row in \ colour 3 STA COMC \ Store the compass colour in COMC \ Fall through into DOT to draw the dot on the compass }